1. Field of the Invention
The present invention relates to a slip control device for suppressing occurrence of slip of driving wheels, a slip control method, and an electric vehicle installed with the control device, the electric vehicle having a pair of right and left wheels driven independently by an electric motor respectively.
2. Description of the Related Art
In a vehicle driven with an engine, right and left driving wheels are connected via a differential and composed such that situations like only the load on the wheel of one side is reduced or slips alone will not happen.
However, in recent years, electric vehicles of light weight and high performance with individual electric motor attached to each driving wheel are prevailing. There is a problem in those electric vehicles that, since the right and left wheel are driven independently, when the load on the wheel of one side decreases or the friction coefficient of the wheel of one side decreases, only the wheel of one side slips or tends to be locked.
A variety of measures has been proposed to deal with the problems. For example, Japanese Laid-Open Patent Application No. 2006-136142 (patent literature 1) discloses an art with which, when traveling condition under which a slip is likely to occur is detected based on the slip ratio calculated from vehicle speed and circumferential speed of wheel, first delay rotation speed obtained by performing first delay processing to the detected rotation speed is used as rotation speed of the induction motor in order to suppress occurrence of slip of the wheel driven by the induction motor.
Japanese Laid-Open Patent Application No. 2000-209705 (patent literature 2) discloses an automatic guided vehicle system for transporting freight, etc. to a destination by automatic driving on a tramway. The vehicle is provided with a master travel device for driving master driving wheels and a slave travel device for driving slave driving wheels. Control is performed such that the torque of the slave drive wheels is suppressed based on the rotation speed difference between the master drive wheels driven under the torque directive from the master travel device and the slave drive wheels driven under the torque directive from the slave travel device.
Further, Japanese Laid-Open Patent Application No. 2006-166572 (patent literature 3) discloses a control device for suppressing slip of an electric vehicle. Control is performed such that a slip is judged according to whether a derivative value of the difference between the circumferential velocity of individual wheel and average of circumferential velocities of all wheels exceeds a reference value of slip or not, and when it is judged that a slip has occurred, driving torque of either the slipping wheel or the mate wheel thereof is reduced.
However, with the method of controlling occurrence of slip according to the patent literature 1, when a slip is detected, the first order frequency is gradually increased by allowing it to pass a first delay filter, so the initial slip can be restricted, but when the slip begins it is difficult to control rotation speed of the wheel once the slip occurs.
With the art disclosed in the patent literature 2, torque control is performed based on the rotation speed difference between the master drive wheels and slave drive wheels. Therefore, it can not be applied to controlling slips of left and right wheels in consideration of difference between rotation speeds of left and right wheels which occur when the vehicle is turned.
With the control device according to the patent literature 3, control is performed by judging occurrence of slip based on a derivative value of difference between the circumferential velocity of individual wheel and average of circumferential velocities of all wheels, the initial slip can be properly judged and restricted. However it is difficult to control rotation speed of the wheel once the slip occurs.
As mentioned above, there are problems in an electric vehicle with motors attached to wheels individually that only the wheel of one side slips or tends to be locked. There have been proposed a variety of slip suppressing control devices for electric vehicles with individual motors for driving each wheel, however, a slip control device for suppressing occurrence of slip with which the vehicle travels stably maintaining good road holding even when traveling on a road where a slip is likely to occur.